An edge fast-ion D-alpha system installed at ASDEX Upgrade

Abstract

An edge fast-ion D-alpha (FIDA) system consisting of lines of sight optimized for edge measurements and a newly designed prototype spectrometer have been installed at ASDEX Upgrade. The system allows for measurements of the full deuterium Balmer alpha spectrum with either good radial coverage or exposure times below 200 μs. It uses a bar, placed in front of the electron multiplying charge-coupled device camera, to partially block the intense unshifted D-alpha light which permits simultaneous measurements of the FIDA and unshifted D-alpha emission. Thus, not only active FIDA radiation present during neutral beam injection (NBI) can be studied but also detailed investigations of passive FIDA light are possible, since the unshifted D-alpha emission contains information on the density of background neutrals. First measurements show significant levels of active and passive FIDA radiation in discharges with large edge fast-ion populations from off-axis NBI heating. In addition, a particularly strong response of the passive emission to edge localized modes is observed, well explained by modifications of the edge-neutral density as well as the fast-ion density.